1. Field of the Invention
The present invention relates to a method, a terminal and a router for detecting a trigger to rerouting, and more particularly to a method, a terminal and a router for detecting a trigger to rerouting for controlling an alteration of redundant routing.
2. Description of the Related Art
In the conventional packet exchange control system (GPRS) standardized under the 3rd Generation Partnership Project (3GPP), a manner of controlling the rerouting of a communicating mobile terminal when it has moved is differentiated with the type of radio channel between the mobile terminal and the base transceiver station during call. That is, available radio channels between the mobile terminal and the base transceiver station are classified into “dedicated channels” on which the volume of communication traffic is heavy and “common channels” on which the traffic volume is light. On the dedicated channel, the radio network controller (RNC) used at the time of initial establishment of communication is used as the anchor. And control to extend the route of data from the anchor (subscriber line extension) is performed. On the common channel, the Gateway GPRS Support Node (GGSN) is used as the anchor, from which routing is switched to the shortest cut from there to the mobile terminal (SRNS relocation (SRNS=Serving Radio Network Subsystem)) is performed. (Reference: 3G TS 25.832 “Manifestations of Handover and SRNS Relocation”.) This rerouting control is performed when the mobile terminal moves from one RNC to another (inter-RNC handover), but this rerouting control is not performed when the mobile terminal moves from one base transceiver station (BTS) to another in the territory of the same RNC (intra-RNC handover). In an intra-RNC handover, the route from the RNC to the BTS can be switched by soft handover, but that at a higher level than the RNC (between the RNC and the GGSN) cannot be switched.
When a mobile terminal performs an inter-RNC handover under the conventional rerouting control by GPRS, optimal rerouting can be accomplished if the radio channel is a common channel, but if it is a common channel, the subscriber line extension entails the occurrence of a redundant portion on the route, resulting in wasteful use of network resources. Moreover, as the choice between subscriber line extension and optimal rerouting solely relies on whether the radio channel is a dedicated channel or a common channel, even if an inter-RNC handover takes place, subscriber line extension may be chosen (if the volume of traffic drops in this state and a shift to a common channel occurs, a change to optimal routing will take place upon that shift), there is a disadvantage that the redundant routing resulting from the movement of the mobile unit cannot be optimized on a real time basis. Furthermore, although SRNS relocation is permitted under the 3GPP standard specifications even when operating on a dedicated channel, essentially SRNS relocation (or subscriber line extension) is a technique that is made feasible by the capability of the network to keep track of whether a mobile terminal has moved from one RNC to another. In a usual IP network, as there are many different topologies including a mesh structure and a tree structure or the like and the structure can usually be altered as desired, it is not realistic for the network to manage the structure, and it is impossible to apply the 3GPP specifications as they are.
FIGS. 7 illustrate how the conventional system works, namely how routing is altered where a mobile communication terminal M under GPRS shifts its position. FIG. 7A shows a case in which the radio channel between the mobile communication terminal M and the base transceiver station is a common channel, and FIG. 7B, a case in which the radio channel between the mobile communication terminal M and the base transceiver station is a dedicated channel. A GGSN 1 in FIG. 7A is a gateway GPRS support node, positioned at the gateway to the network where there is a server or a terminal which is to become a communication partner 8 with the mobile communication terminal M. Communication between the mobile communication terminal M and the communication partner 8 takes place via this GGSN 1. An SGSN 2 is a Serving GPRS Support Node (SGSN), which is connected to the GGSN 1 and is the switchboard nearest to the mobile communication terminal M. An RNC 3 is a radio network controller having functions to control radio resources and to control the handover when the mobile communication terminal M has shifted its position. BTSs 41 to 44 are base transceiver stations, and the mobile terminal carries out communication through connection to one or another of these BTSs.
Where the radio channel between the mobile communication terminal M and the base transceiver station is a common channel as shown in FIG. 7A, the communication path between the mobile communication terminal M and its communication partner 8 is switched from a route R1 to a route R2, which is the shortest cut, with the GGSN 1 as the starting point along with a shift, represented by an arrow Y3, of the mobile communication terminal M. However, where the radio channel is a dedicated channel as shown in FIG. 7B, subscriber line extension takes place whereby, starting from an RNC 31 which was on the communication path when communication was begun, a route R3 is extended toward an RNC 32 and a BTS43, which are the destinations of the shift, in the direction represented by an arrow Y4, of the mobile communication terminal M (a route R4). This system is used to restrain any data loss that may arise when the communication path is switched by a handover, but a more redundant route shown in FIG. 7B will arise, compared with the optimal (shortest) route (the route R2 in FIG. 7A).
Thus under GPRS, even at a timing at which rerouting for optimization is required, the rerouting method is selected solely dependent on the state of the radio channel, resulting in a disadvantage that routing cannot be optimized on a real time basis in response to a handover of the mobile communication terminal M.
Conceivably, this problem could be addressed by either (1) invoking the procedure of change-over to the optimal route upon every handover of the mobile terminal or (2) invoking the same upon an inter-RNC handover.
However, the method of (1) may invoke a wasteful procedure because an intra-RNC handover would need no optimization of routing (the handover would give rise to no redundant route). The method of (2) is unrealistic because managing the structure of a usual IP network, such as the one mentioned above, is difficult to manage and accordingly it is difficult to determine whether or not a given RNC is an “inter-RNC”.